Adds scan unit tests

This change adds much needed unit tests to the scanPackageOnlyLI method
in PackageManagerService. As part of the change, the abi-deriving /
parsing code that is heavily device state dependant has been extracted
into an interface that can be mocked.

Test: atest ScanTests
Bug: 137881067
Bug: 137961986
Change-Id: Ib0b17bffa02570451bd2ad6a649791832c9edd65